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Differential pressure sensor for furnace air filter resistance monitoring ?
QUESTION:I'm looking for a sensor to use for monitoring the air flow resistance
across the filter media in a furnace. Typical values are 0.10 inches
w. c. (= water column ?) when new rising to 0.50 when replacement
should occur. Is there a device whose output is directly (or easily
scaled) in these units ? My thought is to drill a small hole in the
cold air return ducts on either side of the filter and measure
differentially with a sensor. Suggested sensors or circuits ?
ANSWER: At first glance, semiconductor pressure sensors seem
like a good idea. They measure *differential* pressure
directly, and that's what you want. Your medium is
dry air which is compatible with the sensors.
There may be a problem because of wide temperature
fluctuations, but I would have thought that you could
keep the sensor far enough away from the hot part of
the furnace (using thin pipes) to avoid the worst
trouble. The problem, I fear, is the very small
pressure difference you are seeking to measure.
One lb/sq.in (psi) is about two feet of water,
and you seem to be needing an accuracy of around
0.1" water which is only 0.04psi!!!! Not many
semiconductor (silicon strain gauge) pressure
sensors go that low. But in my 1993 SenSym
catalogue I find model SCXL004DN which seems to
fit the bill. It comes ready-calibrated, with
zero error trimmed to about 0.15" water and
span (gain) accurate to about 5%. I guess you
can calibrate-away any zero error, because you
will occasionally have the furnace not firing
and at those times there is sure to be no
pressure drop across the filter - take this
opportunity to calibrate the zero setting.
Try www.sensym.com - I haven't checked out
their website but their data book has loads
of helpful info. They have recently merged
with Foxboro, another major player in
electronic pressure sensors.
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